New Radiation Limits for Spherical Wire Antennas

Considerable work has been published to define the minimum possible antenna radiation Q as a function of the antenna size and gain. Many of the derivations assume an enclosing sphere and deal only with the fields external to this sphere. As a result, the limits tend to be overly optimistic, and realizable antennas generally have Q´s considerably higher than the minimum values predicted by these theories. This paper defines stricter limits that apply to a class of antennas (or scatterers) consisting of any arrangement of conductors on a spherical surface. Energy stored within the sphere is included in the analysis. The minimum Q values are as much as three times the values based only on external fields. Recently published independent experimental data on electrically small antennas in this class reported Q values only slightly higher than the new limits provided in this paper